As a method of estimating a remaining capacity (charging capacity) of a secondary battery, there are a method in which the remaining capacity is estimated by accumulating (integrating) electric-current values and a method in which the remaining capacity is estimated from an open-circuit voltage by using a relation between the open-circuit voltage and the remaining capacity. The former is called a current-accumulation method, and the latter is called an open-circuit voltage method. Each of these two methods has merit and demerit.
The former has a problem with a detection accuracy of current value and an accumulation accuracy of current values. The latter has a problem with a detection accuracy during a charging or discharging of the secondary battery. Hence, in order to enhance an estimation accuracy of the remaining capacity, more accurate one is selected from the these two estimating method in accordance with a state of the secondary battery.
Japanese Patent Application Publication No. 2009-216403 (Patent Document 1) corresponding to US Patent Application Publication No. 2010/0289454 discloses a previously-proposed remaining-capacity estimating apparatus. In this technique, a change of estimate value of the remaining capacity which is caused by the selection between the current-accumulation method and the open-circuit voltage method is restricted when the estimate value has a tendency to decrease during a charging state of the secondary battery or when the estimate value has a tendency to increase during a discharging state of the secondary battery. Thereby, a strangeness feeling of driver that is caused due to a phenomenon in which the estimate value decreases despite during the charging state or a phenomenon in which the estimate value increases despite during the discharging state can be relieved.